No one knows how many are left, although DoC frog ecologist Amanda Haigh said there could be between 5000 and 20,000.

The captive breeding programme was planned after the frog population at a Coromandel monitoring site plunged 88 per cent in the mid-1990s, probably because of the chytrid fungus.

Zoo spokeswoman Jane Healy said the captive frogs could play a critical role in conserving the species. But while the programme got the frogs to lay eggs, only one froglet hatched, and it died.

She said some of the deaths were from a bone disease caused by a lack of nutrition or UV light. The frogs were being treated with UV lamps in their enclosure, which was designed to copy the natural light of the forest.

Meanwhile, the zoo is looking for a new home for its parma wallabies after six died in the past year.